The instant invention relates generally to water jet propulsion apparatus for ships and more particularly to an inlet system having a variable inlet area adjusted by a movable ramp.
In a high speed water jet propulsion system the inlet to the pump is perhaps the most critical component besides the pump because it operates at ship speed and often in a non-uniform velocity field and is thus highly susceptible to cavitation or ventilation. In addition, waterjet efficiency is influenced by inlet system drag and internal losses or energy recovery. Also, pump cavitation is highly dependent on the energy recovery and the outlet velocity distribution of the inlet. Cavitation and internal ventilation of the inlet due to excessive yaw angles can result in degradation of pump and water jet performance, and the cavitation may cause erosion damage.
There are basically two types of inlet systems for water jet propulsion systems, the flush or semi-flush inlet, and the pod-strut or ram inlet. Pod-strut type inlets usually have the inlet opening away from the ship hull on a strut and are required for hydrofoil craft. The flush and semi-flush type inlets have the inlet adjacent to or buried in the hull and are currently favored for surface effect ships.
For most high speed ships, such as the surface effect ships and hydrofoils, significant differences in inflow angles and inlet velocity ratio [(IVR)=average inlet velocity divided by ship velocity] occur at different speeds when fixed geometry inlets are used. Furthermore, to permit cavitation-free operation over these widely varying inflow angles, large inlet leading edge or lip radii or thicknesses are required, which results in a severe drag penalty. If the area of the inlet could be varied such that the inlet velocity ratio (IVR) remained constant over the speed range, the "angle-of-attack" on the lip would remain constant and therefore the leading edge radius of the lip could be small. To vary the area of these flush inlets there are two practical schemes--either move the lip or move the ramp.